A voltage-controlled oscillator (VCO) is a well known electrical component. VCOs are utilized within many synthesizer and tuner circuits, such as those found in TVs and in wireless communication devices. Greater VCO stability with respect to noise sources provides for a more stable VCO output signal, thus for instance, enabling extraction of data from a multiplexed electrical signal. For electronic applications it is desirable to have a broadband voltage controlled oscillator that provides a wide frequency coverage output signal while having low phase noise.
In the prior art, such as in U.S. Pat. No. 6,204,734, entitled “Method and apparatus for extending a VCO tuning range,” an LC resonant tank circuit comprising an inductor and a capacitor is disclosed. This LC resonant tank circuit provides an oscillating circuit within the VCO. The quality factor (Q) of the LC resonant tank circuit is dependent upon the Q of the components making up the LC resonant tank circuit. The LC resonant tank circuit largely determines the phase noise of the VCO, where the higher the Q of the LC resonant tank circuit the lower the phase noise on the output signal. It is known to those of skill in the art that spectral purity of the output signal and RMS phase error are important parameters to consider when designing, for instance, communication systems utilizing VCOs.
A CMOS varactor is a good choice for use as the capacitive element within the VCO LC resonant tank circuit because of its high Q. The varactor is a component that has a voltage dependent capacitance. The voltage dependent capacitance is dependent upon a potential difference applied to the varactor bias and tuning ports, where this voltage dependent capacitance is used to tune the output signal frequency of the VCO. The Q of the varactor is typically larger than the Q of the integrated inductors. Therefore, the phase noise of the VCO is often limited not by the CMOS varactor but rather by the Q of the on-chip inductor. This makes the CMOS varactor a suitable choice for use in the LC tank. Moreover, the CMOS varactor is a device that is readily available in many BiCMOS technologies because it is based on the same fabrication steps as the CMOS device.
Unfortunately, CMOS varactors are tuneable over a limited tuning voltage range, typically the tuning voltage (VT) is varied in the range from a Vbias voltage of Vbias−0.7V to Vbias+1V. Over this VT range the capacitance of the varactor typically varies in a monotonic fashion in response to monotonically increasing VT applied to the tuning port. Design of varactors permits a smaller or larger capacitance change with respect to a same tuning voltage range, however when the capacitance change is larger, noise within VT introduces proportionately more noise into the VCO output signal than with a smaller capacitance changes. If the capacitance of the varactor changes greatly with only small differences in applied VT, then the noise superimposed on the bias or tuning ports will cause commensurate fluctuations in the capacitance and furthermore on the VCO output signal. As such, if a broadband VCO is tuned over a large output signal frequency range then the VCO output signal sensitivity to both internal and external noise sources is increased, especially towards higher output signal frequencies.
Phase noise largely results from internal noise within the LC resonant tank circuit. The internal noise typically originates from resistor and transistor components located inside the LC resonant tank circuit portion of the VCO. External noise is derived from any noise source connected to a VCO tuning port as well as power supply noise originating from a power supply connected thereto. The output signal frequency of the VCO is proportional to KVCO* VT, where KVCO is the slope of the output signal frequency variation with respect to VT variations. As a result, if the LC resonant tank circuit within the VCO has a higher KVCO then the resulting effects of the noise arising from VT are multiplied by the KVCO term. Ideally the VCO has a low KVCO, however the lower the KVCO of the VCO circuit the smaller the broadband response of the VCO. Thus a tradeoff exists between VCO output signal frequency range and noise immunity.
A need therefore exists to provide a VCO circuit that has a wide range output signal with reduced phase noise. It is therefore an object of the invention to provide a broadband VCO circuit that generates a frequency varying output signal with reduced phase noise in response to a tuning voltage applied within a tuning voltage range.